Conventional double-acting pneumatic cylinders typically utilize an opposed pair of V-shaped annular seals between the piston and housing, as illustrated, for example, in FIG. 7 of U.S. Pat. No. 4,242,946. When air pressure is applied in one direction, the legs of the V are urged apart to effect a snug sealing engagement between the piston and housing. When air pressure is applied in the other direction, the contact pressure exerted by the seal on the piston and housing and thus the sliding friction between the seal and housing is minimal.
Other cylinders use annular seals of the O-ring type, which seals have certain disadvantages over V-shaped annular seals, including the fact that they are less durable. Although O-ring seals have been used extensively in single-acting pneumatic cylinders, nevertheless they have previously been considered unacceptable for use in double-acting pneumatic cylinders, particularly at relatively low air pressures, such as 10 to 20 psi. First, replacing the two V-shaped seals in a conventional pneumatic cylinder with two O-rings can present a problem because the O-rings do not permit escape of pressure fluid as do the V-shaped seals, which can result in an undesirable pressure lock between the two O-rings. Further, O-rings are typically subject to substantial variances due to manufacturing tolerances. As a result, if the O-ring is fitted between the piston and the housing with sufficient tightness to ensure that air leakage will not occur at high air pressures, the contact pressure between the O-ring and the housing is relatively high and a relatively high breakaway force is required to initiate sliding movement of the O-ring and piston with respect to the housing. Due to the need for a high breakaway force, the pneumatic cylinder is inefficient or even nonresponsive to relatively low air pressures. If, on the other hand, the O-ring is fitted loosely between the piston and the housing so as to reduce the breakaway force required to overcome the sliding friction between the O-ring and housing, the O-ring will typically be susceptible to low pressure air leaks, due in part to the tolerance-based variances, and will still be inefficient or nonresponsive to relatively low air pressures.
Accordingly, it is an object of the present invention to provide a double-acting pneumatic cylinder utilizing a single O-ring to provide a seal between the piston and housing, which seal is effective under both low pressure and high pressure conditions while simultaneously achieving a minimum breakaway force in terms of the frictional engagement between the O-ring and housing.
It is a further object of the invention to provide a double-acting pneumatic cylinder, as aforesaid, in which the desired seal relationship is effected in a consistent manner regardless of tolerance-based variations in the O-ring.
It is a further object of the present invention to provide a double-acting pneumatic cylinder, as aforesaid, which is durable, substantially maintenance-free, and relatively inexpensive to manufacture.